DNA (Ch. 16)

Brief History Many people contributed to our understanding of DNA –T.H. Morgan (1908) –Frederick Griffith (1928) –Avery, McCarty & MacLeod (1944) –Erwin Chargaff (1947) –Hershey & Chase (1952) –Watson & Crick (1953) –Meselson & Stahl (1958)

Chromosomal theory of inheritance T.H. Morgan 1908 | 1933

What are chromosomes made of? Why did scientists Think that it was The protein? Which component contained the genes?

The “Transforming Principle” 1928 Frederick Griffith –Streptococcus pneumonia bacteria

The “Transforming Principle” Transformation = change in phenotype something in heat-killed bacteria could still transmit disease-causing properties live pathogenic strain of bacteria live non-pathogenic strain of bacteria mice diemice live heat-killed pathogenic bacteria mix heat-killed pathogenic & non-pathogenic bacteria mice livemice die A.B. C. D.

Oswald Avery Maclyn McCarty Colin MacLeod Avery, McCarty & MacLeod Conclusion –first experimental evidence that DNA was the genetic material

DNA is the “Transforming Principle” Avery, McCarty & MacLeod –Purified DNA & proteins from Streptococcus pneumonia bacteria –injected protein into bacteria no effect –injected DNA into bacteria transformed harmless bacteria into virulent bacteria 1944 mice die What’s the conclusion?

Hershey & Chase Alfred Hershey Martha Chase 1952 | 1969 Hershey

Confirmation of DNA Hershey & Chase –classic “blender” experiment –worked with bacteriophage viruses that infect bacteria –grew phages in 2 media, radioactively labeled with either 35 S 32 P –infected bacteria phages 1952 | 1969 Hershey Why use S ulfur vs. P hosphorus?

Protein coat labeled with 35 S DNA labeled with 32 P bacteriophages infect bacterial cells T2 bacteriophages are labeled with radioactive isotopes S vs. P bacterial cells are agitated to remove viral protein coats 35 S radioactivity found in the medium 32 P radioactivity found in the bacterial cells Which radioactive marker is found inside the cell? Which molecule carries viral genetic info? Hershey & Chase

Erwin Chargaff DNA composition: “Chargaff’s rules” –varies from species to species –all 4 bases not in equal quantity –bases present in characteristic ratio humans: A = 30.9% T = 29.4% G = 19.9% C = 19.8% 1947 That’s interesting! What do you notice? Rules A = T C = G

Consider This Brief Film Strip

Structure of DNA Watson & Crick –developed double helix model of DNA other leading scientists working on question: –Rosalind Franklin –Maurice Wilkins –Linus Pauling 1953 | 1962 Franklin WilkinsPauling

Watson and Crick 1953 article in Nature CrickWatson

Rosalind Franklin ( )

It’s Simple, No?

But how is DNA copied? Replication of DNA –base pairing suggests that it will allow each side to serve as a template for a new strand “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.”— Watson & Crick

Models of DNA Replication Alternative models –become experimental predictions conservativesemiconservative Can you design a nifty experiment to verify? dispersive 1 2 P

Semiconservative replication Meselson & Stahl –label “parent” nucleotides in DNA strands with heavy nitrogen = 15 N –label new nucleotides with lighter isotope = 14 N “The Most Beautiful Experiment in Biology” 1958 parentreplication Make predictions… 15 N parent strands 15 N/ 15 N

Predictions 1st round of replication conservative 15 N/ 15 N 14 N/ 14 N semi- conservative 15 N/ 14 N dispersive 15 N/ 14 N conservative 15 N/ 15 N 14 N/ 14 N semi- conservative 15 N/ 14 N dispersive 15 N/ 14 N 2nd round of replication 14 N/ 14 N 15 N parent strands 15 N/ 15 N ✘ ✘✘ ✔ 1 2 P

Franklin Stahl Matthew Meselson Franklin Stahl Meselson & Stahl

Scientific History March to understanding that DNA is the genetic material –T.H. Morgan (1908): genes are on chromosomes –Frederick Griffith (1928): a transforming factor can change phenotype –Avery, McCarty & MacLeod (1944): transforming factor is DNA –Erwin Chargaff (1947): Chargaff rules: A = T, C = G –Hershey & Chase (1952): confirmation that DNA is genetic material –Watson & Crick (1953): determined double helix structure of DNA –Meselson & Stahl (1958): semi-conservative replication

proteinRNA The “Central Dogma”DNA transcriptiontranslationreplication Flow of genetic information in a cell

Science …. Fun Party Time! Any Questions??

Review Questions

1. Tobacco mosaic virus has RNA rather than DNA as its genetic material. In a hypothetical situation where RNA from a tobacco mosaic virus is mixed with proteins from a related DNA virus, the result could be a hybrid virus. If that virus were to infect a cell and reproduce, what would the resulting "offspring" viruses be like? A. tobacco mosaic virus B. the related DNA virus C. a hybrid: tobacco mosaic virus RNA and protein from the DNA virus D. a hybrid: tobacco mosaic virus protein and nucleic acid from the DNA virus E. a virus with a double helix made up of one strand of DNA complementary to a strand of RNA surrounded by viral protein

2. Cytosine makes up 38% of the nucleotides in a sample of DNA from an organism. What percent of the nucleotides in this sample will be thymine? A. 12 B. 24 C. 31 D. 38 E. It cannot be determined from the information provided.

3. In an analysis of the nucleotide composition of DNA, which of the following is true? A. A = C B. A = G and C = T C. A + C = G + T D. A + T = G + C E. Both B and C are true

4. A space probe returns with a culture of a microorganism found on a distant planet. Analysis shows that it is a carbon-based life form that has DNA. You grow the cells in 15 N medium for several generations and then transfer it to 14 N medium. Which pattern in this figure would you expect if the DNA were replicated in a conservative manner? a. d. c. b. e.

5. In analyzing the number of different bases in a DNA sample, which result would be consistent with the base-pairing rules? A. A = G B. A + G = C + T C. A + T = G + T D. A = C E. G = T

6. Imagine the following experiment is done: Bacteria are first grown for several generations in a medium containing the lighter isotope of nitrogen, 14 N, then switched into a medium containing 15 N. The rest of the experiment is identical to the Meselson and Stahl experiment. Which of the following represents the band positions you would expect after two generations? *